Radon is a naturally occurring radioactive noble gas produced by the decay of uranium-238 and radium-226, which are widely distributed in soil and rocks. Radon-222 decays by emission of alpha particles. Radon-222 undergoes four successive decays to radon daughters; polonium-218, lead-214, bismuth-214 and polonium-214. Polonium-214 undergoes further successive decays to Pb-210, Bi-210, Po-210 and ultimately to stable Pb-206.
The radioactive decay products of Ra-222 have a tendency to attach to ambient aerosol particles. Thus, radon and its daughters can enter the lungs with the air we breathe and lodge there. Two of the daughters, Po-214 and 218 decay rapidly, emitting high speed alpha particles (as does radon). If it occurs in the lungs, it can cause cell damage and lead to formation of cancerous cells. Radon concentration is usually expressed as picocuries per liter (pCi/l) or as working level (WL=200 pCi/l=1.3 10.sup.5 MeV of potential alpha energy). Radon in high concentration (i.e., above 0.1 pCi/l) has been found in houses of a number of countries of the world.
Radon can be detected by counting alpha particles in an ionization chamber (Geiger counters), it can also be detected with gamma ray counters by counting gamma rays emitted by its short lived daughters. Such equipment is expensive, bulky and highly sophisticated. Radon in houses is presently monitored by two devices, one is based on adsorption of radon on activated charcoal followed by monitoring gamma radiation emitted by its daughters and the other is based on the etching of latent tracks produced by alpha particles of radon and its daughters in certain plastics.
The activated carbon device requires the use of a highly sensitive, sophisticated, and expensive gamma ray counter. The etch track device requires very long exposure and requires professionally qualified services to provide etching and counting the etched tracks. Neither the charcoal nor the track etch device can be used and results analyzed by an average house owner. There is a need for device for monitoring low concentration of radon that can be used and interpreted by a nontechnical house owner without the need for any expensive equipment and any additional technical services.
Energetic charged particles, such as alpha particles, damage the material along their path. The damaged path, known as track (latent track), is a linear, highly localized region of altered physical and chemical structure compared to the bulk solid. The diameter of the latent tracks is 50-100 Angstroms (0.005 to 0.01 micron). The primary alpha particles from radioactive decay can penetrate approximately 30 microns (micrometer) in a solid or liquid material.
Certain inorganic nonconductors such as silicate minerals, alkali halides, insulating glasses, and organic polymers such as cellulose nitrate, cellulose acetate, cellulose acetatebutyrate, polymethylmethacrylate, poly (bisphenol-A carbonate), and a thermoset polymer of diallyl diethylene glycol carbonate (known as CR-39) are highly susceptible to high energy particles such as Cf-252 fission fragments, nucleon ions of elements, and alpha particles having energy of 0.1 Mev to 200 Mev. The materials that produce latent tracks when exposed to high energy particles are listed in Nuclear Tracks in Solids, Fleischer, Price and Walker, (University of California Press, Berkeley, CA, 1975) incorporated by reference herein. As the rate of diffusion and chemical attack on material in the latent track is substantially higher than the bulk material, the latent tracks can be etched. Particle track etching is a chemical process that preferentially removes the damaged areas and the material surrounding them. Typicaly, strong base solutions are used to etchants for polymeric materials. A list of alpha sensitive polymers, etchants and conditions to be used for etch development of tracks in polymers, glasses and mineral are also given in Fleishcher, Price, and Walker (supra).